The present invention is generally directed to the construction of high-speed coaxial cables. More particularly, the present invention is directed to the structure and construction of individual cables having a pair of signal wires and is particularly directed to such cables which exhibit uniform and controllable resistance under a variety of field conditions. Even more particularly, the present invention is directed to a high-speed cable which is capable of operation at speeds in access of 25 MHz and which can be used in connection with pluggable connectors for insertion into circuit board connection sockets. The present invention is also directed to the construction of high-speed cables which may be employed in conjunction with cable connectors exhibiting densely packed linear spacing of less than 0.025 inches. Additionally, it is noted that the present invention is directed to the construction of a high-speed cable which is usable in a high-speed emulation engine and, in particular, which is usable for direct connections to and from printed circuit board connector sockets which are disposed around chips on a printed circuit board.
The construction of high-speed cables is difficult because the speed, tolerance and variability requirements are strict and severe. For example, a ribbon cable such as that shown in U.S. Pat. No. 5,256,082, issued to Yaegashi et al., is completely inappropriate for the high-speed cabling applications required by the present day emulation engines. In particular, it is noted that a ribbon cable is very difficult to route. It cannot be bent in sideways directions easily, and if indeed it is bent, there is a degradation in the uniformity of its electrical characteristics, particularly its electrical resistance. However, for purposes of the present invention, it is important that the electrical characteristics of the cabling be precisely controllable and consistent from one cable unit to the next and also that the characteristics be relatively constant over time particularly when the cabling is possibly subjected to handling and movement, even the kind that occurs during installation and set-up of a new machine in which the cable is deployed.
For example, FIG. 1 in the patent to Yaegashi et al. shows coaxial conductors embedded in an insulative material. However, in spite of the fact that the drawing therein illustrates a somewhat substantially constant-looking spacing for the conductors, in practice, such ribbon cables are difficult to manufacture with the stable electrical characteristics which are required for high-speed operation. In particular, the amount of insulative material which may be present between the various coaxial conductors is highly variable. For example, in the present invention, the applicant sought to create a cable with a variation in impedance of less than 10 percent in a 50 ohm cable. Not only is the impedance value difficult to produce consistently in a ribbon cable, but bending and routing of ribbon cables will distort the alignment of the conductors and will produce undesirable and unpredictable changes in the impedance value.
Another significant disadvantage of ribbon cabling for high-speed use is the variability that is introduced into the electrical properties of the cable when it is cut and/or when it is trimmed for purposes of exposing conductors for attachment to the other components in a high-speed connector or circuit. Furthermore, cutting of a ribbon cable such as is described by Yaegashi et al. introduces even further distortions and variations in the insulative material which is disposed between the different coaxial conductors in the ribbon. Furthermore, the ribbon cable arrangement shown in FIG. 1 of the patent to Yaegashi et al. does not provide good protection against nicking of the central signal conductor during cutting and stripping operations. This is particularly true at the left end of the ribbon cable shown therein. Thus, while it is desirable not to nick any of the conductors during cutting operations, it is seen that ribbon cable is, in fact, particularly sensitive to this kind of damage during cutting and stripping operations which are necessary to both cut the cable to its desired length and also to strip off material to expose the inner conductors.
Furthermore, it is noted that the alignment of the coaxial conductors in the patent to Yaegashi et al. will, in fact, be somewhat haphazard even as the ribbon cable comes from a manufacturer. This non-alignment situation is only aggravated when the cable is cut, stripped or bent. In particular, the planarity of the alignment suggested in their figure is, in fact, not preserved during ribbon cable cutting and trimming operations. In short, ribbon cable is impractical for use as a high-speed cable in emulator or mainframe computer systems. While ribbon cable is an appropriate conductive link in relatively low-speed personal computer applications, for truly high-speed applications, ribbon cable is an inappropriate and inadequate choice of conductor.